(a) Field of the Invention
The present invention relates to a variable aberration lens system which can be used not only as a general photographic lens but also can be a soft focus lens by varying the spherical aberration.
(b) Brief Description of the Prior Art
The performance required as of a soft focus lens used mostly for photographing a portrait or the like is to make a picture covering a soft kind of large halo around a core small enough. For that purpose, it is desirable to make the ratio of a sum .SIGMA.I of the fifth degree spherical aberration coefficients to a sum .SIGMA.I of the third degree spherical aberration coefficients .SIGMA.I/.SIGMA.I&gt;150 so that the higher degree spherical aberration may be generated to be larger than the lower degree spherical aberration and the contrast of the halo with the core may be larger.
One of the methods which can attain the above mentioned object is shown in Japanese published unexamined patent application No. 141223/77. That is to say, by so forming the cementing surface a of the third lens group in FIG. 1 that the center of curvature may be near a stop S, the radius of curvature may be smaller and the difference between the refractive indexes in the front and rear of said cementing surface may be very small, it has been successful to generate a spherical aberration of a higher degree on this cementing surface. However, in this method, as the spherical aberration is generated on the cementing surface, when the spherical aberration is made smaller and is used for a general photographic lens, a smaller stop aperture will have to be selected. Therefore, such method has a defect that a field of view of the finder tends to become dark.
Another method of attaining the above mentioned object is shown in FIG. 2 of U.S. Pat. No. 1,463,132 and in U.S. Pat. No. 4,124,276. That is to say, an air space is provided between two nearly concentric surfaces which are concave against the stop so that a higher degree spherical aberration may be generated by the cancellation effect through these two surfaces. In this method, by making the radii of curvature of the above mentioned two surfaces smaller, .SIGMA.I/.SIGMA.I&gt;150 can be made and, by varying said air space, the amount of the spherical aberration can be controlled. However, if the radii of curvature of the above mentioned two surfaces are made smaller, when said air space is varied so that .SIGMA.I may be the same as in a general photographic lens, .SIGMA.I will remain to have a large value, a high resolving power and high contrast will not be obtained and such lens will not be able to be used as a general photographic lens. On the contrary, when the radii of curvature of the above mentioned two surfaces are made larger, the spherical aberration is corrected to be as small as of a general photographic lens and said air space is varied to be of a soft focus lens, it has been impossible to make .SIGMA.I/.SIGMA.I&gt;50.